Add newline to each generated bash script in rbe_test.go.

"exec format error" showed up on Mac when running the rbe_test.go.
The generated bash scripts did not have a newline at the end of
the file.

Bug: b/162600461
Fixes: b/162600461

Test: "m blueprint_tools" on Android Mac builder.
Change-Id: I425fa5f428733b6569c3fcdaa312023f9761d020